Beginning with the 12G-SDI (I)

Most transmission and video facilities use data cables. In fact, some professionals say that these applications will eventually only consist of data cables.

Steve Lampen *

4K is a term commonly used to describe the resolution of the video screen that is about 4000 pixels. That's about four times the resolution of HD in its highest version: 1080p. The broadcast version of 4K, called UHD (ultra-high definition), has a resolution of 3840 pixels by 2160 lines. DCI (Digital Cinema Initiative), the 4K version of Hollywood, has a resolution of 4096 pixels by 2160 lines. Both have a clock speed of close to 12 GHz, hence the 12G-SDI.

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Several years ago, we created a RG-6 video cable (1694A) that transported HD to more than 370 feet (113m). But when the 3G-SDI arrived on the scene, that video signal - also called 1080p / 60 or 1080p / 50 - was twice the bandwidth of high definition (HD), which reduced the distance capabilities of the 1694A to 78m .

Although it has changed over the years, the magic distance for video cables today is 100m (328 feet). I've always wondered where that number came from. Is not that limiting the distance of data cables like Category 5e, 6 or 6A? How does this apply to the video cable? But then it occurred to me: Most transmission and video facilities use data cables. In fact, some professionals say that these applications will eventually only consist of data cables. At this time, many installations have a hybrid design with data and coaxial cables, so it may make sense for the coaxial cable to follow the same rule.

That's when we decided to create the first cable specifically designed to carry 3G-SDI signals up to 100m: 1794A - and we did it about five years ago. It was a cable slightly larger than 1694A. Today, HD almost governs the world of video, however, the RN 7 cable that we created did not end up being a super seller. Even then we knew that the next step for video cables would involve 4K.

But would it be possible to transmit 12G-SDI signal via coaxial cable? Most people said no.

An important note about the digital cliff
Now is a good time to point out that the distances cited in this blog are not based on field tests. These are calculated values ​​based on a formula that first appeared in SMPTE ST-259M to avoid what is known as the "digital cliff". The farther we go on a cable, the closer we get to the digital cliff. The image looks good, so we really do not know how close we are to stop receiving it. But when you install a cable that is too long, you no longer get any image. The receiver chip does not see enough signal (or the noise and reflections mess up the signal), so it does not get anything. The difference in length from a perfect image to no image could be only a few feet.

If you know the clock frequency, or the data rate, of the application, we can determine to what extent the signal can go safely and maintain an image along the cable. In any digital data system, the actual data can not exceed a frequency of half the clock. This is called the Nyquist limit. Originally, the formula for SD-SDI was -30 dB (attenuation) to ½ of the clock frequency. Using this formula, digital signals could easily be sent hundreds, even thousands of feet before reaching the distance -30dB. Then, with the change to HD, SMPTE ST 292 was written with a more conservative formula of -20 dB at ½ the clock frequency. That means you can not go that far. This safety distance was very conservative.
 
How far does it go? Over time, HD cable performance continued to improve, and we consistently received this type of feedback from customers: "I went twice as far as your distance chart" Some customers even said they could go three times further. Not only did the cable improve, but also the connectors and chips that sent and received the signals.

When it seemed that 4K would eventually become standard, Belden pushed the SMPTE standards group to change the distance formula for these applications. We proposed a new formula and got our wish: -40 dB to ½ of the clock frequency. This means that, for an 12 GHz cable, the attenuation must not be greater than -40 dB to 6 GHz (½ clock frequency of 12 GHz).

For the 6K 4 GHz version, the formula would be -40 dB to 3 GHz. When comparing the data in the following distance table, this is the reason why it seems that the numbers sometimes do not make sense. The formulas (shown at the top of each column) continue to change. This is just an excerpt from our table of Recommended Transmission Distances. And it shows our first cable 4K, 4794R.

There are quite a few new things to emphasize in this graphic. The first is the column for SMPTE ST-425, which covers 12 GHz quad-link for UHDTV1. This was the original 12 GHz delivery system, which divided 12 Gbps into four cables. In that case, each cable carries 3 Gbps / 3 GHz for a cable that already exists. But, with the new formula (-40 dB to ½ of the clock frequency), they go further than in the previous SMPTE ST-424 standard, even though they are the same cables that have always been used.

If the cables under the formulas -30 dB and -20 dB can go two or three times the distance shown, where is the cliff for the 12G-SDI cable - especially in the SMPTE ST 2082-1 column? The actual location of the cliff is influenced by the quality of the installation, the chosen connectors, the equipment, the chips, the connectors and everything else on the line (connection panels, connection cables and connectors, adapters, feed-through, etc)
 
A new cable for signal transmission 12G-SDI

We recently launched a new cable for 4K / UHDTV (12G-SDI): Belden 4794R. This coaxial cable is the first designed specifically for the 4K single-link UHD video cable in the broadcast market for the 12G-SDI signal transmission. The 4794R coaxial from Belden for 4K / UHDTV (12G-SDI) offers superior performance, easier installation and reduced weight and space compared to quad or dual links.

If you have questions or comments, please contact me at steve.lampen@belden.com or with our partners in Latin America (juan.cortes@belden.com)
 
* Steve Lampen joined Belden at 1989, being currently product manager for broadcast. Before Belden, he worked as an engineer in radio, film and television. It has an FCC license, SBE and BICSI certifications. At 2011 he was named "Best Educator" by the Society of Broadcast Engineers. His book "The Audio-Video Cable Installer's Pocket Guide" is published by McGraw-Hill. Follow him on his blog www.belden.com/blog
 

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